Abstract
Hydrogen (H2) shows great potential in reducing greenhouse gas emissions and improving energy efficiency due to its environmentally friendly nature and high gravimetric energy density [1]. It can be generated via electrochemical water splitting based on the hydrogen evolution reaction (HER). It is well known that Pt-group metals (PGMs) are excellent catalysts for HER, but their broad adoption is limited by high cost and scarcity. Recently, two-dimensional (2D) molybdenum disulfide (MoS2) is regarded as a promising alternative to PGMs due to its large surface area, rich active sites, and ideal hydrogen adsorption energy [2]. However, its practical application is hindered by the intrinsically low electrical conductivity arising from the semiconducting nature of2H phase MoS2[3]. On the other hand, 2D Ti3C2 MXene with high electrical conductivity, excellent hydrophilicity, and large interlayer distance has been intensively investigated in energy storage devices lately[4]. Compared with charge-neutral graphene, MXene exhibits a negatively charged surface due to the existence of numerous surface functional groups (-OH, -O, -F, etc.), which not only enhances the dispersion of MoS2 precursors but also promotes MoS2 nucleation, making it a superior template for MoS2 synthesis. Nevertheless, undesired oxidation of MXene occurs in aqueous solutions [5], reducing the overall catalyst stability.
To address the above issues, we employed a one-step solvothermal method using DI water/DMF as bisolvent and constructed metallic 1T phase-enriched MoS2/MXene composite as HER catalyst. The advantages of using bisolvent lie in twofold: (i) suppress undesired oxidation and thus preserve high conductivity of MXene framework, and (ii) improve MoS2 electrical conductivity by inducing 2H to 1T phase transition. The introduction of metallic 1T phase MoS2 is triggered by ion intercalation. Specifically, during the synthesis, both ammonium molybdate (Mo precursor) and DMF can act as abundant sources of NH4+ which can intercalate into MoS2 layers. This process stimulated charge imbalance between Mo3+ and Mo4+ and led to the S plane sliding [6]. As a result, crystal structure distortion and therefore phase transformation of MoS2 occur along with interlayer distance expansion. To further improve the catalyst conductivity, carbon nanotubes (CNTs)were introduced into the binary composite as crosslinks to bridge the 2D islands. As a result, a low overpotential (169 mV) and Tafel slope (51 mV/dec) along with the highest turnover frequency (7 s-1 at -0.23V vs. RHE) and an ultralong lifetime (72 hours) was successfully achieved. The origin of the outstanding HER performance of the ternary composite can be ascribed to: (i) the prevention of 2D layer restacking as well as the enlarged surface area due to the 2D/2D MoS2/MXene integration and ion intercalation. This will promote the contact between electrolyte and catalyst, resulting in an increased hydrogen ion adsorption; (ii)the vertical growth of MoS2 flakes on MXene template which increases the exposure of MoS2 edge planes, maximizing the total number of active sites; (iii) the synergistically enhanced conductivity because of the formation of hybrid 1D/2D conductive network via the integration of 1T-phase metallic MoS2, conductive MXene backbone with suppressed oxidation along with the CNT crosslinks, minimizing the charge transfer resistance at the electrode/electrolyte interface. This work demonstrated an effective strategy for low-dimensional material structure-property engineering with the aim of optimizing the HER performance which will shed light on the development of the next-generation PGM-free HER electrocatalysts.